Semiconductor devices, such as integrated circuits, are generally fabricated by the repeated application of a photolithographic process. A photosensitive material is applied to the surface of a semiconductor substrate, which may include layers of dielectric materials, polycrystalline silicon, and various metals. A radiation source is used to transfer the pattern of a lithographic mask or reticle onto the photosensitive material. The patterned photosensitive material is then used as a lithographic mask to process the semiconductor substrate or at least one of the layers on the substrate. The lithographic mask may be used, for example, as an etch mask or as an ion implantation mask.
For example, when fabricating MOS transistors one step involves forming contact holes or openings through various layers of material to the gate electrode and source and drain regions. To form such contact holes or openings, a layer of photoresist can be spin coated on the surface of a layer of dielectric material. The photoresist layer is patterned by exposing certain parts of the photoresist layer to light. Notably, there are two types of photoresist: negative photoresist and positive photoresist. When a negative photoresist layer is used, the parts of the negative photoresist layer which are not exposed to light can be washed off by wet chemical treatment; when a positive photoresist layer is used the exposed parts of the positive photoresist layer are washed away.
Once the photoresist layer has been patterned, the remainder of the photoresist layer can serve as a lithographic contact mask. The lithographic contact mask covers most of the underlying dielectric material layer, but also includes small round openings at the locations of the future contacts. The portions of dielectric material layer underlying the small round openings can then be selectively etched in a manner that is highly unisotropic to the lithographic contact mask. Etching transfers the pattern of the lithographic contact mask into the portions of dielectric material layer that underlie the small round openings in the lithographic contact mask to thereby produce contact holes. The contact holes can have a diameter of approximately 70 nm. These contact holes are subsequently filled with a conductive material to form electrical contacts to contact regions of the gate electrode and source and drain regions.
As the number of individual devices incorporated in a semiconductor device increases, there is a growing need to decrease the minimum feature size, that is, the minimum width, the minimum space between individual elements of the devices, the minimum widths of holes or vias, and the like. As the minimum feature size decreases (e.g., as lines and spaces on the integrated circuit become smaller and more closely spaced), the wavelengths necessary to resolve the patterns becomes shorter and shorter. When the minimum feature size is less than the wavelength of the radiation source, it becomes increasingly difficult to adequately resolve the features because of diffraction and interference effects. Optical distortion causes a loss of the anticipated one-to-one correspondence between the image on the mask and the image created in the patterned photosensitive material. As such, there is a practical limit to the size of the features which can be resolved in the photoresist layer using conventional photolithography techniques. For example, as the minimum feature size reaches 45 nm or smaller, it becomes increasingly difficult to resolve small critical dimensions (CDs) for contact holes in the photoresist layer. This is particularly true for small contact holes that open to source and drain regions since these contact holes have to be squeezed between adjacent gate electrodes. In many cases, it becomes difficult to etch contact holes with a vertical profile, and the etch process must be designed to stop on contact regions of the gate electrode yet still open up contact holes to the source and drain regions which reside below the gate electrode. In other words, because the source and drain regions are below the polysilicon gate material over the gate electrode, the contact holes to the polysilicon gate material are generally not as deep as the contact holes to the source and drain regions. As such, the height of the contacts which need to be formed can vary and have different heights because contact points which need to be contacted lie in different planes.
While extreme ultraviolet (EVU) lithography and electron-beam lithography may be used to pattern photoresist with small feature sizes, such processes are complex and very expensive.
Accordingly, it is desirable to provide improved methods for fabricating semiconductor devices that have small feature sizes. In addition, it is desirable to provide methods for forming and etching small features, such as contact holes, through material layers, particularly where the features being etched have a critical dimension which is smaller than the wavelength of light, and hence can not be patterned in photoresist using conventional photolithography techniques. It is also desirable to provide improved methods for forming contact holes which have a relatively straight or vertical etch profile. It is also be desirable to provide improved methods for forming contact holes which exhibit the ability to stop etching at different depths or levels in a selective manner. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.